Method of recovering acid in the production of cyclonite



Aug. 2, 1949. G. F WRIGHT El AL 2,478,164

" METHOD OF RECOVERING ACID IN THE PRODUCTION OF CYCLONITE 2 SheetsSheet1 Filed Dec. 30, 1944 [n msniors: Gemye FPVFz'y/z? and C arlA. Wmk Z 6 rAug. 2, 1949.

c; F. WRIGHT ET AL ,4 METHOD OF RECOVERING ACID IN THE PRODUCTION OFCYCLONITE Filed Dec. 50, 1944 2 Sheets-Sheet 2 m h m 5 a 6 04 e d W; m Ve 0 0 no 5 u 2U .v b e PM 5 3 d w w 2b .W m 04 j VJ -we mmw Low W. P 4 io a a T W a e 4 5 9 0. w w w w w 0 m m m M W N M Q fi fi b X P m R NNSQPatented Aug. 2, 1949 UNITED STATES FATE-N T OF FlCEi M'ET-HQD 9FREGOVERING ACID PRQDUCTIQN F CYGLONITE George-F. Wright, Toronto;Ontario; and-Garl-A; Winlrler, Montreal, Quebec, Ganada; assignors toThe HonoraryAdvisory Counoil f'or Scientific and-Industrial Research,Ottawa, Ontario,- Can ada, a corporationof theDominion-of Canada;

Application December 30, 1944', Serial No. 570,812

(Ch .2 Gib-541) l0-Glaimst 11 The present inventionrelates to acidrecovery and 1 more. particularly to .therecovery of acetic acid fromspent reaction liquors resulting from theman-ufacture' of certainexplosives especially cyclonite.

In theomanufactu-re; ofpyclonite, either: by the process described inBachman application, Ser. No; 495,078filed July- 16,- 1943, or by the:related process described in Schiessler and Ross application,- Ser. No;444,254, filed May 23,1942, now Patent No. 2,434,230, -January-6; 1948,-there'is produced asa' byeproducti-a. spent reaction: liquorcontaining .a considerablelquantity of acetic acid which may amount toas much as approximately 3.5 lbs. of acetic acid per pounduof cycloniteproducedinlthe process The recovery of acetic acid from such spentreaction liquors istherefore essential if these processes .are-to ba-commercially attractive. However,- recovery operations with such spentreaction liquors are hazardous, firstly because ammoniumnitrate-containing organic matter constitutes a'high: explosive andsecondly because the.1iquors contain residual. amounts. of cyclonitewhich is knownto betanexplosive of great power and brisance.

The object of the present inventionis to provide anew, improved and safeprocessior the recovery 0t acetic acid from crude acidcontainingammoniumnitrate and residual explosive nitramines,

A further object is to provide a method of the characterldescribed whichdestroys the hazardous components of spent reactionliquors resultingfrom the manufacture of cyclonite produced by processes involving theuse of acetic anhydride and ammonium nitrate.

Another 'pbi'r-ectis an acetic acid recovery "process applicabletocrude-acetic acid containing ammonium nitrate and cyclonitawhichprevents the accumulationlof explosive at any point in the systemor theretention of the explosive in the recovered acid.

Still another object is to providea process that enablesthe recovery ofacetic acid without the addition of a reagent which would remain. in therecoveredacid tothe detriment of acid quality.

A further object, is the provision of an acidrecovery process which maybe carried out continuously' in apparatus constructed with a minimum ofstrategic materials, producing recovered acetic acid-of over 99% purity;

Other objects and advantages of the process Will be apparent-astheinvention is hereinafter more fully described;

In accord-ance with the'present inventionpit has been-found that theacetic acid present in spent mother liquors of the type characterized 5abiive maybe-recovered byfirst'fuming oii the'crude acid in the presenceof a-small amount of-sulfu-ri'c acid and thereafterdistilling the fumedoff *reaction liquor at a temperature sufficiently high to destroyany-organic explosivematerial contained therein. Bythe foregoingprocess'it'is" possible to recover acetic'acidof over99'.5% purity. Ex-

plosive hazards areminimized during therecovery operation bythe'destruction" of the ammonium nitrate during the fume-off step, andof the cyclonite and"related compounds"during the distillation step;

In carrying out the processor the present invention it is desirablefirstly to control the fumeofi andsecondly to'efiect 'ra'pid'andcomplete destructionof the organic explosive remaining in the fumedoff"'liquors, without the use of a re- ,agent whichwould 'remainintherecovered acid to the detriment of acid quality. The presentinventionattains these objects by means whichw'ill hereinafter bedescribed."

In*order* moreclearly to describe the invention the process will bediscussed with reference to the I; :THE FUME-OFF STEP In 'themanufa'ctureof cyclonite by the-Bachmann process, hexamine' (preferablydissolved in glacial acetic acid) nitric acid, ammonium nitrate and-acetic anhydride 1 are brought together in a reaction vessel at atemperature'abov'eabout 35 :C. Irr the manufacture of cyclonit'e by theSchiessler and Ross pro'cess; paraformald'ehyde {is treated withammQnium-nitrateand acetic anhydride at' a temperature above about 35 C.The reaction mixture resulting from either of these processes'is-aslui'rWconSisting of What may convenient-ly ha -termed an-hydridecyclonite" suspendedin a mother liquor which contains among otherthings, ammonium nitrate and acetic acid. For purposes of convenience,the mother liquor from both of these processes may be designated asanhydride cyclonite mother liquors or reaction liquors.

The process of the present invention is based in part upon the discoverythat, if the undiluted filtrate or mother liquors from the aboveprocesses are heated in the presence of a small amount of sulfuric acidto a threshold temperature of about 95 to about 100 C., a violentreaction ensues during which the color of the liquid suddenly changesfrom straw color to deep brown and large volumes of nitrogen oxide gasesare evolved. During this fume-01f reaction, the ammonium nitratetogether with a substantial portion of the nitric acid and the unstablenitramine by-products of cyclonite are decomposed. The violence of thereaction is due to the gas evolution which causes considerable foam. Thefoam however is unstable and disappears rapidly as soon as gas evolutionceases. The fume-off reaction is exothermic and the temperature of thereaction mixture rises to about 106 C. in the absence of added heat.

(a) Sulfuric acid concentration during fume-o1? The fume-off reaction isself-sustaining provided sufficient sulfuric acid is present in themixture. The sulfuric acid concentration necessary to obtain aself-sustaining fume-off depends in part upon the composition of thefiltrate or of the liquor. Usually, however, the minimum amount ofsulfuric acid varies from about 5% to about by weight of the fume-offmixture.

(b) Nitric acid concentration during fume-0 7 If the anhydride cyclonitemother liquor or filtrate contains an appreciable amount of nitric :acidand ammonium nitrate, the fume-off is much more violent in operation. Itis therefore sometimes desirable to reduce the nitric acid and ammoniumnitrate content of the filtrate prior to ammonia to convert practicallyall of the nitric acid to ammonium nitrate which is sparingly soluble incold (ZS-30 C.) neutralized mother liquor but fairly soluble in themother liquor containing appreciable amounts of nitric acid. When theresulting ammonia-treated filtrate is fumed off very little nitrogendioxide is formed but considerable nitrous oxide gas is evolved at asteady rate at a temperature of about 106 C.

In this connection it may be mentioned that if all the nitric acid isneutralized with anhydrous ammonia some difficulty may be experiencedoccasionally in sustaining the fume-off reaction. To avoid thisdifliculty it is preferable to neutralize the major proportion but notquite all of the nitric acid, leaving less than 1% of the nitric acid,for example, 0.5% unneutralized. Ammonium nitrate is only sparinglysoluble in mother liquors containing this small amount of nitric acidand the mixture fumes off smoothly and continuously at about 106 C.

Either before or after the treatment with anhydrous ammonia, but in anyevent prior to the fume off, any large quantities of solid cyclonitepresent in the reaction mixture are of course removed by filtration. Theammonium nitrate may be readily removed from the separated cyclonite bypassing hot water through the filter cake. In order to accelerate therate at which the ammoand water.

nium nitrate is leached from the filter cake it is preferable to blowlive steam therethrough.

(0) Temperature during fume-017 Temperature is an important factor incontrolling the violence of the fume-off reaction. If the temperature ishigh enough to initiate rapid fume-off (about to about C.) the heat ofthe reaction is sufiicient to raise the liquid temperature to about 106C. This temperature can be maintained without further outside heatprovided fresh mother liquor-sulfuric acid mixture is added at anappropriate rate. If outside heat is supplied during fume-off until thetemperature of the liquid rises to 112 C., the evolution of nitrogendioxide may cease for reasons at present unknown and the evolution mayor may not be restored by lowering the liquid temperature to the properrange between about 95 C. and about 106 C. However, Once the fume-offreaction has ceased due to overheating, the restoration of the fume-offis sometimes very difficult to brin about. It is therefore important tocontrol the temperature of the fume-off reaction so that it takes placebelow about 112 C., and preferably so that it takes place in theneighborhood of 106 C.

Inasmuch as the constant addition of fresh mother liquor-sulfuric acidmixture assists in maintaining the fume-off at an easily controlledconstant rate, continuous operation during fumeoff is preferred althoughbatch fume-off may be carried out where desired. However, in batchfume-off of even a small volume of mother liquor the reaction issometimes quite violent and for this additional reason continuousfume-01f is much to be preferred.

A convenient method of carrying out the fumeoff is to start with a smallvolume of mother liquor maintained at fume-off temperature in a suitablereaction vessel and then to add sulfuric acid or oleum to the motherliquor until fumeoff begins to take place. Thereafter additional motherliquor containing from about 5% to about 10% by weight sulfuric acid maybe constantly added to the reaction vessel, the feed being preheated ata temperature (about 50 C.) sufficient to maintain the fume-off reactionmixture at a temperature in the neighborhood of 106 C. By providing thereaction vessel with a draw-01f tap, the fumed off liquor may bewithdrawn from the reaction vessel at a rate sufficient to maintain afairly constant level in the reactor.

(d) Impurities in the famed 017 liquor The composition of the fumed 01fmother liquor depends of course upon the composition of the originalmother liquor. The ammonium nitrate and hydrolyzable organic impurities,such as N- acetylpolymethylene nitramines, are removed during thefume-01f but approximately the original amounts of residual cycloniteand homocyclonite remain in the fumed-off mother liquor as wereoriginally present in the mother liquor prior to the fume-01f reaction.Traces of nitric acid may also be present in the fumed-off motherliquor, together with larger amounts of sulfuric acid Generally, thewater content will be considerably higher in the fumed-off mother liquorthan in the original mother liquor; the average increase in Watercontent being about 1.6% regardless of the original water content of themother liquor prior to fume-off. Apparently the water formed duringfume-off results almost ennitrate in the mother liquor.

t. i =5 1, .2. L12 t .w wflrflzt "dehydration" reaction involving mq i reaud nit oapiqerror: or THE" FUMED- OFF Mo 1 r IQQUQR F, V ('a)Destruction of leyclom'teond homocyclomte pointed "out above," the iiuiii'e ofi" reaction notdestroy the residual cyclonit'e and homokfyion'ite' containedih the mother liquor. 'In acooifdahe with a furtheraspectof the present in vn'tion'it' has beenfound that this residualcyclon'ite and homocyclonite maybe completely def strbyed duringdistillation of the acetic'acidfpro: 'vided the fumed-off mother liquoris distilled from a sulfuric acid-acetic acid mixturecontain ing arelatively high-percentage of sulfuric acid, .maint'ained at atemperature within the range of about 125 to about 150 C.

The rate of destruction of cycloni'te in mix- THER tures of acetic acidand sulfuric acid is dependent partly-upon the temperature of themixture and partly upon the sulfuric acid concentration. Fig. 2 is agraphical representation of the rate of destruction of 'e ucmte infumed-off mother liquorsulfuric acid iniX-tures at the boiling point ofthe mixtures, From this graph it will be apparent that 100% of thecyclonite may be destroyed in less than- 2 minutes provided the sulfuricacid content of the boiling'mixture is above about by weight; Fig. 3 isa graphical representation sulfuric acid by volume will be above about130 C Accordingly in effecting the distillation of the acetic" acid inaccordance with the present invention,' complete and rapid destructionof the residual cyclonite and homocyclonite is insured by introducingthe'fumed-ofi mother liquor into a mixture of sulfuric acid and aceticacid of 'at least 20% by weight sulfuric acid strength main tained atatemperature above about 120 110140" C In order to ensurevery rapidand'complete destruction of the residual organic explosive, thefumed-oft mother-liquor is distilled through a mixture consisting ofabout 40% sulfuric acid and 60% glacial acetic acid. Furthermore,inasmuch as the 'fume-off reaction is preferably carried outcontinuously it is desirable to effect the distillation in an apparatusadapted for continuous operation, 1 Y I do) Compositiomofihedistillate... it 1,

The distillate'hroducedin accordance with the present invention consistsof acetic acid or above 95% urityfthe precise composition of'thedistilla'te'being tat-item ionof the residue'm'aintained in thestillduring"distillation." The distillate contains afvarylng but usuallyvery small amount of water whrenus derived from' three possible sources,namaym the Water in the fumed' off mother dependent partly upon'thesulfuric acid K liquor resulting from the decomposition of amgreatlyreduces'the solub'ilityof the ammonium duceci bythe econ'd sourceisresidue in the stil-liattains a composition of about 14.2% sulfuricacid;2.6% water and 83.-2-%acetic acid, it -appears -thatsulfuric acidexercises no appreciabledehydrating action on the acetic' acid. Fromabout 15 to about 35 sulfuric acid content thedehydrating efiiciencyorthe sulfuric a-cidincreases to a -maximum giving a distillate of"progressively higher acetic acid strength.= After a maximal dehydratingefficiency at about 35 sulfuric acid in the stillresidue,'- theefficiency decreases sharply. Accordingly the highest qual--- ityaceticacid distillateis obtained at a maximum dehydrating concentrationof'about 35% sulfuricacid in the still 'mixture. However, thetotalrecoVe-ry of" acetic acid is notmaXimal under these;

' to about 40% sulfuric acid in the still residues;

represent convenient operative ranges.

*As an illustration of the composition ofthe distillate obtainable inaccordance with the proc-,- ess of the present invention, an experimentwas. carried out with 7. l. of fumed oil mother liquor: containing about5.4% sulfuric acid. Thisfumedi oiT mother liquor was distilled through130 cc; of'a mixture consisting of 40% sulfuric racidandl 60% glacialacetic acid using a reflux ratioof 0:1,. The distillate consist-odoraceti acid of over 99 purity. In some instances very smallamounts ofaldehydes maybe present.v Where it is desired to reduce the aldehydecontentof the recovered acid, the acid may be digested .witha smallamount of 'aniline and then distilled. This practice, however, isusually not necessary.

III. 'ILLusrRA mvE EXAMPLE In order to disclose the nature of thepresentinvention still more clearly a typical example will hereinafter bedescribed with reference to Fig. I or" the drawing. It-sliould beclearly understood that this is done solely by way of illustration-andis notintended to delineate the invention in its broadest aspects.

Inthis example-it will be assumed that the cyclonite was produced inaccordancewith the Bachmann process by reacting hexamine dise solved inacetic acid with ammonium nitrate-and nitric acid-inthe presence ofacetic anhydride at a temperature Off about 35 to about C; After-thereactantsxare brought together in the reaction vessel, the resultingslurry is withdrawn from the reactor. and cooled to about 25 to about30" .C; This undiluted but cooled slurry from -the Bac'hmann process isthen treated with nearlystoichio-metric amounts of anhydrous ammonia in.

an amount suificient to reduce the nitric acid.

content to about 0.5%, thereby reducing the: solubility of the ammoniumnitrate in the reaction liquor. a After this ammonia pretreatment, theslurry i filtered,'the cyclonite and other solid materials: remaining inthe filter bed being worked up by hot water wash and by passing livesteam through the-filter Ibed, prior to the purification procedure"described in the above identifiedBa-chmann application. y

Afterthe'removal' of the solid cyclonite' thefiltrate is then ready forthe recovery process ractically': inala terable i n quantity; -The waterproduced-from the" third source varies with the isulfuric acidconcentration of -the -sti1l-;resiclues;- Until-the:

7 which may conveniently be carried out in the apparatusdiagrammatically illustrated in Fig. 1. The apparatus as shown includesa fume-off chamber Ii) (preferably constructed of Duriron) a series ofboiling tubes l I, an entrainment separator l2 and an acetic acidcondenser [3.

In starting the unit in operation the crude acid (i.e., the motherliquor from which the cyclonite has been filtered) together withsulfuric acid, is introduced into the fume-off chamber l through thecharge lines 2!) and 2|, the proportions being so adjusted that theresulting mixture contains from about 5% to about sulfuric acid, thehigher concentration being generally more satisfactory in starting upthe continuous system. A sufiicient amount of this mixture is introducedthrough the charge lines to fill the hydraulic legs 22 and 23 andpartially fill the pot Ill. The mixture in the hydraulic leg 22 is thenheated by passing steam through the steam jacket 24'. After fumes beginto evolve in the fume-off pot In the crude acid and sulfuric acid(preferably preheated to about 50 C.) are constantly fed through thecharge lines 20 and 2i in a ratio of roughly 95 parts by weight of crudeacid to 5 parts by weight of sulfuric acid, the feed rate being adjustedto maintain a substantiall constant liquid level in the fume-off chamber80. During the fume-off operation the temperature of the liquid in thefume-off pot I0 is maintained as closely as possible at about 106 C. Theevolved gases and vapors pass upwardly through the conduit 25 into acondenser 26, any condensate being returned to the fume-ofi pot throughthe trapped line 21; uncondensed vapors and gases pass through the vaporoutline 29 to the fume stack (not shown).

The fumed-off reaction mixture overflows through the hydraulic legs 22and 23 into the upper boiling tube II. The boiling tubes consist of anumber of steam jacketed horizontaltubes disposed above one another andso connected as to form a continuous flow path. Each of the horizontaltubes is provided with suitable dams 28 so that each horizontal tube ismaintained partially full of liquid. The fumed-01f mother liquor flowsinto one end of the upper tube and thence in sequence through the lowerhorizontal boiling tubes. The lowermost boiling tube is provided with atake-off tap 30 through which residual sulfuric acid-acetic acid may bewithdrawn. The temperature of the liquid in the uppermost boiling tubeis maintained at about 125 C. while that of the lowermost boiling tubeis held at about 140 C.

In order to minimize back pressure effects, a vapor line 3i connects thevapor space of the horizontal boiling tubes to the sulfuric acidentrainment separator l2.

As the vapors from the boiling tubes ascend through the entrainmentseparator [2 (which is preferably packed with Raschig rings or othersuitable surface material) the entrained sulfuric acid or other liquidin the vapors is separated, the vapors ascending to the condenser i3.The condensed acetic acid thence flows into the recovered acetic acidstorage tank 32.

Inasmuch as the residual acid flowing from the lowermost boiling tubemay contain 60% acetic acid or more, it is preferable to recirculatethis acid to the fume-off vessel, this being accomplished by the returnline 33 which connects with the recycle storage tank 34. The residualacid in the storage tank 34 is forced by the pump 35 through the returnline 36 to the overhead 8 storage tank 3'! whence it flows through thevalved line 38 to the fume-off pot ID.

The boiling tubes II are preferably steamjacketed (not shown) in orderto supply heat to the liquid undergoing distillation.

By way of recapitulation, in using the apparatus described withreference to Fig. 1, assuming that the process is .to be used inconnection with the manufacture of anhydride cyclonite, the cycle ofoperations is preferably as follows:

(1) Treat the undiluted reaction mixture (consisting of a slurry ofcyclonite in the undiluted reaction liquor) with anhydrous ammonia untilnearlyv all of the nitric acid has been neutralized,

leaving approximately 0.5% of the nitric acid unneutralized. Theresulting slurry will then consist of cyclonite, acetic acid, ammoniumnitrate, about 0.5% unneutralized nitric acid, and other reactionproducts, impurities, etc.

(2) Filter the cyclonite and other solid products.

(3) Mix the filtrate with sulfuric acid, forming a solution preferablycontaining about 9 to 10% sulfuric acid in starting up the acid recoverysystem, the filtrate and the sulfuric acid entering the fume-off vesselpreferably being prewarmed to about 50' C.

(4) Heat the liquid in the hydraulic leg 22 until fume-off is initiatedand thereafter maintain the temperature of the liquid in the fumeoff potat about 106 C.

(5) After the fume-off has started, continue charging to the fume-01fvessel It], prewarmed (50 C.) crude acid and sulfuric acid in theproportions of about by weight crude acetic acid and 5% by weightsulfuric acid.

(6) Permit the fumed-01f liquor to flow into the boiling tubeevaporator, maintaining the temperature in the uppermost tubes of theevaporator at a temperature slightly above the boiling point of aceticacid (for example at C.) and the lowermost tubes of the evaporator at atemperature not exceeding about C.

(7) Condense the acetic acid vapors passing through the entrainmentseparator I2, in the condenser 13.

(8) Continuall withdraw residual acid (about 40% sulfuric, 60% aceticacid) from the tap-off line 30, recycling either all or a portionthereof where desired to the fume-off vessel It.

It will be apparent to those skilled in the art that many variations maybe made in the foregoing detailed procedure without departing from thespirit and scope of the invention. We therefore intend to be limitedonly in accordance with the following patent claims.

We claim:

1. A method of recovering acetic acid from mixtures containing ammoniumnitrate, nitric acid, acetic acid and residual nitramine explosives,which comprises heating the reaction mixtures in the presence ofsulphuric acid to effect fume-off, and thereafter distilling thefumed-off reaction, mixture from a residual acid containing about 15 toabout 40% sulfuric acid.

2. A method of recovering acetic acid from mixtures containing ammoniumnitrate, nitric acid, acetic acid and residual nitramine explosives,which comprises heating the reaction mixture at a temperature from about100 to about 112 C. in the presence of sufficient sulfuric acid toeffect fume-off, and thereafter distilling the fumed-off reactionmixture from a residual acid comprising about 15 to about 40% sulfuricacid and about 60 to about 85% acetic acid, said re- 9 sidual acid beingmaintained at a temperature of about 120 to about 140 C.

3. The method of claim 2 wherein said sulfuric acid is present to theextent of about 5% to about by weight.

4. The method of claim 2 wherein said fumedoff reaction mixture isdistilled from a mixture of sulfuric acid and acetic acid of about 35 toabout 40 sulfuric acid strength.

5. In the recovery of acetic acid from residual reactions mixtureliquors obtained in the manufacture of nitramine explosives, the methodof working up the reaction mixture which comprises treating theundiluted reaction product with anhydrous ammonia to reduce the nitricacid content thereof to below about 1%, separating the solid reactionproducts from the mother liquor, heating the mother liquor from whichthe solid has been removed to a temperature between about 95 to about112 C. in the presence of sufficient sulfuric acid to effect a fume-off,distilling the fumed-off mother liquor from a residual acid comprisingabout to about 40% sulfuric acid and about 85% to about 60% acetic acid,maintained at about 120 to about 140 C., and condensing the acetic aciddistilled from said mixture.

6. The method of claim 5 wherein the separated solid reaction productsare treated with hot water to leach out the water soluble and waterhydrolyzable impurities therein, whereby to recover cyclonite.

7. The method of claim 5 wherein said sulfuric acid is present to theextent of about 5 to about 10% by Weight.

8. The method of claim 5 wherein said residual acid is utilized in thefume-off of further quantitles of said crude acid.

9. A method of recovering acetic acid from a crude acid containingammonium nitrate, nitric acid, acetic acid, and a nitramine explosive,which comprises mixing said crude acid with sulfuric acid to form amixture containing about 5 to about 10% sulfuric acid, heating saidmixture at a temperature of about -112 C. to effect a fume-off of saidmixture Withdrawing a portion of the fumed-off mixture and distillingthe same fro-m a residual acetic acid-sulfuric acid mixture having aboiling point between about and about 140 C.

10. A method for the recovery of acetic acid from residual reactionliquor obtained in the manufacture of cyclonite, which comprises:treating the undiluted reaction mixture with sufficient anhydrousammonia to reduce the nitric acid content thereof to below 1% by weight,separating the solid reaction products including solid cyclonite fromthe reaction mixture: heating the mother liquor remaining afterseparation of said cyclonite in the presence of about 5% to about 10% byweight sulfuric acid, at a temperature sufiicient to produce a fume-01f,whereby to destroy the ammonium nitrate and nitric acid content of saidmother liquor; distilling the acetic acid from the fumed-off motherliquor from a body of about 15 to about 40% sulfuric acid and about 85%to about 60% acetic acid, maintained at a temperature of about C. toabout 0., whereby to destroy any residual cyclonite contained in saidmother liquor, condensing the acetic acid distilled from said mixture;and mixing the residual undistilled sulfuric acid-acetic acid mixturewith further quantitles of said mother liquor prior to fume-off anddistillation thereof.

GEORGE F. WRIGHT. CARL A. WINKLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,609,393 Bader Dec. 7, 19262,355,770 Wyler Aug. 15, 1944

